Legislation restricting the amount of pollutants that may be emitted into the atmosphere is becoming increasingly strict. One category of pollutant that is legislated against by inter-governmental organisations throughout the world is unburned hydrocarbons (HCs). Unburned HCs of varying composition are typically present in exhaust gases produced by various types of mobile or stationary engine, such as spark ignition engines, compression ignition engines and combustion turbines.
Natural gas (NG) typically comprises a hydrocarbon (HC) gas mixture and small amounts of carbon dioxide (CO2), hydrogen sulfide (H2S), water vapour (H2O) and nitrogen (N2). The main component of NG is methane (CH4), but relatively small amounts of ethane (C2H6), propane (C3H8) and other hydrocarbons are usually also present. There is interest in using natural gas (NG) as a fuel for engines, particularly in the form of either compressed natural gas (CNG) or liquefied natural gas (LNG). For vehicular applications, the use of CNG as a fuel is typically preferred over LNG because CNG generally has both lower production and storage costs compared to LNG.
Engines have been produced that are purpose-built for using NG as a fuel. It is also possible to modify existing internal combustion engines to use NG. Engines are known that can use NG as a fuel in a variety of ways, such as alone (e.g. a “dedicated” NG engine) or in conjunction with another fuel (e.g. a bi-fuel engine) where the engine may be run on one fuel at a time or both fuels may be used in unison. When NG is used as a fuel, the exhaust gas produced by the engine contains significant quantities of methane (so-called “methane slip”). Methane is a potent greenhouse gas (GHG). Also, in comparison to other HCs that are typically present in an exhaust gas, methane and ethane are difficult to catalytically oxidise over a catalytic convertor, especially in the presence of excess oxygen, such as in the exhaust gas produced by lean burn NG combustion engines.
Commercially available oxidation catalysts for treating methane/ethane typically comprise palladium (Pd) or platinum (Pt) and palladium (Pd) supported on alumina (Al2O3). These catalysts have to be operated at high temperatures (e.g. >500° C.) to achieve reasonable methane conversion efficiency. Other oxidation catalysts have been investigated but they often suffer from poor thermal stability. There is also the problem that many methane oxidation catalysts are sensitive to poisoning by sulfur.
The oxidation of methane using noble metal based catalyst has been reviewed by P. Gélin and M. Primet (Applied Catalysis B: Environmental, 39 (2002), 1-37). Oxidation catalysts comprising Pd supported on a zeolite are described. The authors concluded that the “use of zeolitic supports did not bring any evidence of improved catalytic activity in methane oxidation of Pd catalysts”.